The invention is directed to an electrical contact piece made of a silver-metal oxide material as contact coating or overlayer, a carrier made of copper or iron and an intermediate layer. Thereby the intermediate layer can serve as solder for joining the contact coating-carrier, or as a solderable layer on the contact piece for soldering to the carrier by means of a conventional solder.
Silver-metal oxide material is employed to a large extent because of its favorable industrial switching properties as contact material in switching having a high number of switchings at low or average current load. The most important members of this composite material-group are silver-cadmium oxide, silver-tin oxide, silver-zinc oxide, and silver-indium oxide.
On account of the high metal oxide content the contact coatings according to the present state of the art are not directly solderable to metallic contact carriers. Therefore for the use as contact material in electrical switching devices the contact coating must be made solderable. For this reason the contact coatings are provided on one side with a layer of a good solderable or weldable metal or metal alloy.
Thus it is known for example, to oxide only one side internally (about 70% of the sheet thickness) of the workpiece consisting of silver alloys and to use the non-oxidized layer as a solderable surface (German AS No. 1033815). However, the process is only usable with silver alloys which can be internally oxidized and are still sufficiently ductile in the oxidized state so that they can be converted into the finished part. With silver alloys which are not sufficiently ductile in the oxidized state the shaping takes place before the oxidation. Thereby the starting material for the production of a solderable layer is plated with silver. The contact coatings are then oxidized as finished parts (German AS No. 2334160 and German AS No. 1803502). However, the silver layer at long oxidation times loses adhesive strength and finished contact coating is detached from the contact carrier in operation. Besides there are formed in the middle of the sample inhomogeneities, such as Kirkendall pores, which under certain circumstances result in a reduced life for the contact coating.
Silver-metal oxide contacts produced by powder metallurgy can be provided with a solderable silver layer by pressing and sintering in common (German AS No. 1187333). However, this process is expensive and only usable with silver-metal oxide contact coatings produced by powder metallurgy.
It is known from German AS No. 1090484 to provide silver-cadmium alloys on one side with a silver solder layer and only then to oxidize internally. However, the process has the disadvantage that it is only usable with internally oxidizable contact materials. Furthermore, metal physical conditioned phenomena here lead to industrial process difficulties which later bring with them working problems in the soldering of the contact piece (e.g. increase of the working temperature of the solder).
In German AS No. 1232282 there is described a soldering process in which the contact surface to be soldered is preliminarily soldered with a tin-lead soft solder and subsequently the contact is soldered with a customary silver hard solder to the contact carrier. The disadvantage of this process is that the parts to be joined must be heated for several minutes to temperatures above the working temperature of the silver-hard solder in order to guarantee a trouble-free joining. If the contact carrier consists of hardened alloys, then the hardening effect can be lost.
Besides it is known (German AS No. 2365450) to back up the silver-metal oxide work material with a solderable silver-copper alloy having 10 or 28% copper and wetting the joining material and subsequently to solder with the contact carrier. Also copper-silver alloys having 8 to 40% silver have been proposed for this purpose (German OS No. 2941423). However, these solderable layers also have not proven sufficiently adhesive with all silver-metal oxide work materials.
Especially the recent preferably used Ag/SnO.sub.2 contact work materials having increased SnO.sub.2 content permit poor rolled plates. Furthermore, the plated silver has a lower mechanical strength compared to the dispersion hardened contact work material. Through the thermomechanical stresses in the switches there can form already at a low number of switches a tear in the silver which leads to the scaling off of the contact coating.
Compared to silver, plated on copper has an increased strength but in the heat treatments required in the course of the process of production there arise in the transition region between the solderable layer and the Ag/SnO.sub.2 layer porous edges. In the course of the switching stresses there runs along the pores a tear which already at a low number of switches likewise can lead to sealing off of the contact coating.
Reactive solders which contain phosphorus (German OS No. 2438922 through reduction of the SnO2 likewise produce pore formation. In the switching experiments this type of presoldered or soldered contact coatings fall off at corresponding loads through separations in the region of the damaged zone.
Therefore, it was the problem of the present invention to find an electrical contact piece made of a silver-metal oxide work material as contact coating, a carrier made of copper or iron and an intermediate layer which also furnishes a sufficient adhesive strength of the contact coating to the contact carrier at a high number of switches.